Commercial vehicles, such as for example tractor trailers, often utilize air brake systems. A typical air brake system is capable of performing a service brake function and a park brake function. Service braking or normal braking refers to actuating the vehicle's brakes by depressing a brake pedal to deliver compressed air to a brake actuator. Service braking is used to slow the vehicle or bring the vehicle to a stop. Once stopped, the driver may wish to apply the park brakes to prevent the vehicle from rolling. Typical vehicle air brake systems utilize spring brake chambers to perform the park brake function. Spring brake chambers utilize spring force to engage the brakes and hold the vehicle stationary. Air pressure is used to disengage the brakes by compressing the spring.
Referring to FIG. 1, tractors typically include a compressor 10 that provides the compressed air used to operate the tractor's brakes, as well as a trailer's brakes. The compressor provides compressed air to one or more reservoirs 12 on the tractor. When a brake pedal 14 is depressed, the brakes (not shown) on the tractor are engaged. In addition, depressing the brake pedal sends pressurized air over a service line 16 to the trailer to engage the brakes 17 (FIG. 2) of the trailer. This pressurized air is referred to as pressurized service air in this application. The tractor also includes a trailer supply valve 18. The trailer supply valve 18 selectively provides air from the tractor to the trailer over a supply line 20. This pressurized air is referred to in this application as pressurized supply air.
Referring to FIGS. 1 and 2, when the trailer supply valve 18 is in a first state (typically a depressed actuator 19), pressurized supply air is applied over the supply line 20 to the trailer. The pressurized supply air is routed by a spring brake valve 21 to fill spring brake chambers 22 to release the spring brake and fills trailer reservoirs 24 with pressurized air. When the supply valve is in a second state (typically a pulled-out actuator 19), the supply of pressurized supply air to the trailer is cut off and the spring brake valve 21 and/or the trailer supply valve 18 vents the spring brake chambers to apply the spring brakes.
When the brake pedal 14 is depressed, pressurized service air is communicated to a relay valve 26. The relay valve, in turn, supplies pressurized air to service brake chambers 30 to engage the brakes. When the brake pedal 14 is released, the relay valve 26 exhausts the service brake chambers 30 to release the brakes.
Existing spring brake valves 21 are typically configured operate in only a single mode where priority is given either to the service brake (i.e. fill the reservoir before releasing the spring brake) or to the spring brake (i.e. releasing the spring brake before the reservoirs are filled to a normal operating pressure).